Let's make a data table to understand how the forces and torques have to add to zero so we don't get any rotating, collapsing bridges.

Force	L.a.d.	pos. or neg.	Torque,
RA=	0	+	0
FB=10kN	1	-	-10kN
RC=	4	+	4RC

So add the torques in that column of the data table and set them equal to 0:

Solve the above equation for R_c and put that value both in the table and on the diagram. Then use that value in the equation below to solve for R_A:

Put your values for R_A and R_c both in the data table and on the diagram. Then answer these questions:

Question #1: Why does the lever arm distance for the torque resulting from the reactant force at point C have a value of 4?

Question #2: Why does the lever arm distance for the force (F_B) due to the weight of the truck have a value of 1?

Question #3: Why is the torque due to the reactant force at point A equal to 0?

Question #4: Why is F_B negative?

Question #5: Why is R_C positive?

A R = B F = 10KN R=